Often, a film-like anti-reflection structure for improving visibility is provided on the surface of a display, for example, a computer or the like. A method for preventing reflection by closely disposing a plurality of fine convex protrusions on the surface of a transparent substrate (transparent film) has been proposed as this anti-reflection structure. The so-called “moth-eye structure” principle is applied in this method. Moth-eye structures seek to prevent reflection by continuously varying the refractive index with respect to incident light in a thickness direction of the substrate, thereby sudden fluctuations in refractive index at the interfaces are eliminated and anti-reflectivity is obtained.
Such an optical element can realize high antireflective performance, provided that the refractive index with respect to incident light can be continuously varied. As such, in principle, anti-reflection structures are not limited to fine convex protrusions and may also be fine concave protrusions. It is preferable that a ratio of height or depth of the fine convex protrusions or concave protrusions to a width of the structure (hereinafter referred to as the “aspect ratio”) be great. This is because when the aspect ratio is great, the refraction index changes will be gradual and high antireflective performance can be obtained. On the other hand, with larger aspect ratios, stability of the anti-reflection structure provided with the convexity or concave protrusion structure is lost. Thus, there is a problem in that it is difficult to maintain the shape of the structure. There is also a problem in that larger aspect ratios lead to increased complications when fabricating the structure. For example, these types of fine shapes can be fabricated by nanoimprinting or other methods in which a mold is used. However, there is a problem in that, when the aspect ratio is high, the mold becomes clogged with resin or the like when transferring the convex protrusions or concave protrusions from the mold.
As such, many lines of research are being pursued to obtain high antireflective performance without increasing the aspect ratio. For example, Patent Literature 1 describes improving the antireflective performance of an optical element by providing two or more steps in convex protrusions or concave protrusions. If two or more steps are provided in the convex protrusions or concave protrusions, discontinuous interfaces will occur in the interfaces where the refractive index of the moth-eye structure continuously varies. These discontinuous interfaces reflect a portion of the incident light. Patent Literature 1 suggests that the reflected light can be reduced by causing the light reflected at the discontinuous interfaces to interfere with each other. However, the optical element described in Patent Literature 1 has not provided sufficient antireflective performance.